Paper recycling has, in recent years, become a more important and attractive option to disposal of waste paper by deposition in landfills or by incineration. It has been a common practice for many years to make paper, especially tissue, from recycled paper. Typically, the waste paper is supplied to a hydropulper where the paper is pulped with caustic, dispersants and large amounts of water to form a slurry of long fibers, short fibers, fines and fillers.
Copending application Ser. No. 08/118,186 discloses processes for utilizing waste paper, preferably office waste that is printed with laser print, photocopier print, or other inks, and magazines that have a coated surface. The waste paper is pulped with water, caustic and surfactants to produce a slurry containing paper fibers, paper fines and fillers. After several washing steps, the slurry passes through a wire washer which has screens that separate papermaking fibers from fines and fillers. The papermaking fiber stream, also referred to as the "accepts stream", is directed to a cleaning and de-inking step and then to a conventional papermaking machine for processing into paper. The filtrate from the washer continues through the process to a flotation clarifier where the suspended solids (fines and filler) are concentrated as a flotate and clarified water is removed for reuse in the process.
The flotate is further passed through a belt press where the water content is further reduced. The product from the belt press is in the form of a wet, particulate filter cake. The wet filter cake then passes, by means of a screw conveyor, to a pin mixer. The filter cake is broken up in the screw conveyor and the pin mixer. The pin mixer reduces the size of the particles as compared to the size of the particles that are discharged from the screw conveyor. The wet particles or granules are then sent through a conveyor dryer to produce dry absorbent particles or granules having a moisture content less than 10% by weight. The resulting particles or granules have an irregular shape and have good absorbent characteristics.
There are many processes available for recycling waste paper to recover clay, fibers and other by-products. Many of these processes utilize a hydropulping or fiber releasing step in which a low consistency water slurry is formed. Some of these processes produce particles or granules that are gray in color. Other processes produce particles or granules having a color which is dependent on the color of paper being produced in papermaking machines whose flow systems are part of these processes.
It would be desirable to mix the dry granules from the above process with soil as a soil conditioner. The granules help to keep soil loose and aerated, as well as help retain moisture. However, since the granules from the pin mixer are typically gray in color, mixture with soil as a conditioner does not provide a product which appears rich and fertile. Thus, the product does not provide the potential of high percentage blending with soil. Likewise, use of such particles for other products such as absorbents or agricultural chemical carriers may not be desirable due to their color.
It is possible to dye the particles from recycling processes. However, due to factors such as the high surface area and the absorbency of the particles, it is extremely costly to obtain significant color change since large quantities of dye are necessary. Further, dye retention is a problem due to interference from commonly used flocculating polymers. Accordingly, improved methods for recycling waste paper to provide products of a desired color are needed.